The invention relates to a method for measuring the filling level in containers by means of electro-optical distance measuring employing the reflected wave principle. Further, the invention relates to an apparatus for performing the method.
Methods for measuring the filling level in containers are known in many modifications. Thus, German Patent Publication (DAS) 10 75 852 discloses a measuring device in which the filling level is ascertained by the observer who must compare two reflected beams by coinciding two images. This coinciding of the two images is accomplished by a mechanical adjustment of the mirror angle on a scale.
According to German Patent Publication (DAS) 11 75 514 a method is known in which light is introduced into the liquid from a light conducting body depending on the filling level in the container. The light quantity which exits at the upper end of the light conducting body represents a measure for the filling level.
Both of the just described devices are not suitable for general applications and are not capable to measure with an exactness which keeps the tolerance to one mm.
German Patent Publication (DOS) 22 08 931 discloses an electro-optical indicator for the ascertaining of the level in containers. The indicator comprises a measuring instrument, as well as means for controlling the current flowing through the instrument as a function of the liquid held in the container. The control means comprise a light source with a constant light or luminous flux as well as a light sensitive element which is influenced by the light source through a bundle of optical fibers, whereby the light flux impinging upon the light sensitive element and travelling through the bundle of optical fibers, is controlled by an indicator or transmitter of the liquid level in the container.
This device, according to German Patent Publication No. 22 08 931, is also not capable of measuring with a 1mm accuracy because this known device is not capable of eliminating error values caused by environmental conditions.
German Patent Publication (DOS) 22 35 329 discloses an arrangement for the exact level measuring in which an optical-electronic measuring device with a digital data output is employed. A closed measuring tube filled with an inert gas is connected to the measuring device. One portion of the measuring tube extends into the container and is connected to the cover member in a force locking manner so that the remaining portions of the measuring pipe extends outside of the container and have degrees of freedom in two axes. A flat ring float with a permanent magnet is arranged around the portion of the measuring tube inside the container. A ball supported measured value indicator is supported by the magnetic field which is established inside the pipe by the ring float. The measured value indicator is provided with a centrally arranged reflector having a pyramid shape.
The apparatus according to German Patent Publication No. 22 35 329 is rather involved in its equipment because of the many inputs and outputs. In addition, disturbing influences which occur as a result of surrounding conditions and which falsify the measurement, are not completely eliminated. A substantial disadvantage of the known apparatus is seen in that the reference point at the bottom of the tank which is necessary for the filling level measurement, cannot be held in a stable condition. In addition, it is necessary that the measuring apparatus itself is precisely fixed and adjusted in its position because each position variation may change the measuring distance from the measuring instrument to the reflector and would thus enter into the measured result. This must be taken into account.